The present invention relates generally to a cryocooler-cooled superconductive magnet, and more particularly to such a magnet having a cryocooler coldhead which can be easily removed and replaced.
Superconducting magnets may be used for various purposes, such as to generate a uniform magnetic field as part of a magnetic resonance imaging (MRI) diagnostic system. MRI systems employing superconductive magnets are used in various fields such as medical diagnostics. Known designs include cryocooler-cooled superconductive magnets wherein the cryocooler coldhead is supported by the superconducting coil assembly through a cryocooler penetration assembly. Typically the cryocooler coldhead is mounted in a cryocooler sleeve assembly with a vacuum space independent from the cryostat vacuum, which allows for cryocooler coldhead removal and replacement. However, before removing the cryocooler coldhead, the superconducting magnet has to be slowly warmed up in a process that may take weeks. Removal of the cryocooler coldhead without slowly warming up the superconducting magnet would lead to warm room-temperature air entering the sleeve assembly from which the cryocooler coldhead was removed. Such warm room-temperature air would contact the cold surfaces within the sleeve assembly forming an ice ball of frozen water and air on such surfaces including those which had been in thermal contact with the removed cryocooler coldhead. These contaminated surfaces would require time-consuming cleaning before a replacement cryocooler coldhead could be installed. Additionally, removal of the cryocooler coldhead without slowly warming up the superconducting magnet would lead to the room-temperature air transferring a heat load to the magnet quenching its superconductivity.
Another known superconductive magnet design permitted the substitution of a second cryocooler coldhead for a first cryocooler coldhead during superconducting magnet operation. However, neither of the two cryocooler coldheads could be removed and replaced without a lengthy warm-up period for the superconductive magnet without contaminating the cryocooler penetration assembly and quenching the magnet.
A further known superconductive magnet design had a sleeve assembly between the cryocooler coldhead and the superconducting coil assembly. The design described the first stage of the cryocooler coldhead as being rigidly attached to the sleeve (such as by brazing) which would require breaking such rigid attachment to replace the cryocooler coldhead.
What is needed is an MRI magnet design which permits a cryocooler coldhead to be replaced without a long magnet warm-up period and without having to break and later having to re-establish a brazed (or the like) rigid attachment of the cryocooler coldhead to a sleeve (or other cryocooler penetration) assembly.